Conventional two-stage isolators generally comprise two single-stage isolators in series to increase isolation and bandwidth. Each single-stage isolator further comprises one non-reciprocal polarization rotator and two birefringent optical wedges. Thus a total of four birefringent optical wedges are needed. As a result, the conventional isolators are more difficult to assemble due to the fact that there are more interfacing surfaces between these four wedges, and that the relative orientations of the optical axes among these wedges have to be precisely aligned to guarantee good isolation within certain bandwidth. In addition to the requirement of achieving good optical isolation, the alignment processes must also simultaneously control two other functional parameters, i.e., adjustments and control of the polarization mode dispersion (PMD) and polarization dependent loss (PDL). The reliability is often degraded due to the difficulties in the assembling processes. All these difficulties limit the usefulness and functionality of the conventional isolators.
Therefore, a need still exists in the art of manufacturing and designing the fiber-optic communication system to provide an improved isolator design and configuration to overcome the difficulties encountered in the prior art. It is desirable that the new isolator configurations and method of manufacturing can provide isolators of reduced number of required optical wedges and therefore reduced number of interfacing surfaces between these wedges. It is also desirable that the process of compensating for PMD can be decoupled from the process of controlling isolation and PDL. Thus the above-mentioned difficulties associated with assembly and alignment of optical wedges and limitations of insertion loss and polarization dependent losses can be resolved.